1. Field of the Invention
The present invention relates to a flexible printed board provided with a rust-resistant polyimide insulating layer. It also relates to a novel polyamic acid that yields a rust-inhibiting polyimide as a result of an imidation treatment.
2. Related Art of the Invention
Flexible printed boards in which polyimide insulating layers are formed directly on metal foil such as copper foil without an interposed adhesive are fabricated by methods in which polyamic acid varnishes obtained by the addition polymerization of aromatic diamines (such as paraphenylenediamine) and aromatic dianhydrides (such as pyromellitic dianhydride) in solvents such as N-methyl-2-pyrrolidone are applied to the copper foil and dried to yield a polyamic acid layer (polyimide precursor layer), and the product is heated and imidated at 300 to 350xc2x0 C. to form a polyimide insulating layer.
Such polyamic acids are also used as starting materials for moldings, films, insulating varnishes, adhesives, and the like.
The presence of carboxyl groups in the polyamic acids is disadvantageous in that when copper foil is coated with polyamic acid varnishes in the manner described above, the copper foil surfaces are corroded and discolored, and copper ions are produced, causing electrical migration in the flexible printed boards.
Imidazole-based rust-inhibitors (for example, Adekastub CDA-1, manufactured by Asahi Denka) are commonly added to the polyamic acid varnishes used in the manufacture of flexible printed boards.
Conventionally used rust-inhibitors are disadvantageous, however, in that they dissolve poorly in polyamic acid varnishes, create excessive blooming on the polyimide surface when heated to a high temperature during imidation, and reduce the adhesive strength of polyimides in relation to copper foil. In addition, the rust-inhibitors are scattered during imidation, forming a resinous substance that deposits on the imidation equipment or flexible printed board products and contaminates them.
An object of the present invention, which was perfected in order to overcome the above-described shortcomings of prior art, is to provide a flexible printed board whose polyimide insulating layer has adequate adhesive strength with respect to copper foil and is free of the problems associated with electrical migration even in the absence of rust-inhibitors such as those promoting blooming and resinous substance formation during the imidation of polyamic acids.
Another object of the present invention is to provide novel polyamic acids which are as precursors of such polyimides.
The inventors perfected the present invention upon discovering that the stated objects can be attained by using specific imidazolyl-diaminoazines as the diamine components of polyamic acids, introducing azine structures into the main chains of the polyamic acids, and introducing imidazole rings into the side chains thereof.
Specifically, the present invention provides a flexible printed board in which a polyimide resulting from the imidation of a polyamic acid obtained by the addition polymerization of diamines and acid dianhydrides is formed as an insulating layer on a metal foil, wherein this flexible printed board is characterized in that the diamines include imidazolyl-diaminoazines represented by the formula 1; 
(where A is an imidazolyl group represented by the formula 1a, 1b, or 1c; 
R1 is an alkylene group; m is 0 or 1; R2 is an alkyl group; n is 0, 1, or 2: R3 and R4 are alkylene groups; p and q are each 0 or 1; and B is an azine residue, diazine residue, or triazine residue).
The present invention also provides a novel polyamic acid, obtained by the addition polymerization of diamines and acid dianhydrides, wherein said polyamic acid is characterized in that the diamines include imidazolyl-diaminotriazines represented by the formula 10; 
(where A is an imidazolyl group represented by the formula 1a, 1b, or 1c
R1 is an alkylene group; m is 0 or 1; R2 is an alkyl group; n is 0, 1, or 2; R3 and R4 are alkylene groups; and p and q are each 0 or 1).
These and other objects, features and advantages of the present invention are described in or will become apparent from the following detailed description of the invention.